The present invention refers to a device for the treatment of flat materials, more particularly for embossing and/or calendering foils, comprising at least one first and one second embossing roll, one of these rolls being driven and one of these rolls being free-wheeled, between which rolls the flat materials are passed under pressure in order to produce a pattern. A device of this kind is e.g. known from U.S. Pat. No. 5,007,271 or U.S. Pat. No. 5,598,774 of the same applicant. In particular, the flat materials in question are bands or strips one face of which is coated with metal, generally with aluminum, and whose support consists of paper or another fibrous material, or foils, e.g. of synthetic materials, of metal, or of compound materials.
The above-mentioned embossing devices are e.g. used in the packaging industry for the purpose of embossing packing materials while seals, devices, etc. may be stamped at the same time. In this context, the term embossing means providing the surface with a fine pattern in the millimeter or submillimeter range. Strip or sheet packing materials of this kind are e.g. used for the packaging of tobacco products, cigarettes, foods, chocolates, chewing gums, and the like.
The devices mentioned in the introduction have been successful and still are as long as the embossed foils, more particularly packaging foils, are mainly composed of aluminum or mainly of easily foldable paper. In the production of cigarette packages, for example, these packaging materials serve the purpose of wrapping up a counted number of cigarettes in order to be subsequently inserted in a box.
In order to be able to meet the increasingly severe environmental prescriptions, the embossing materials have been modified in the sense that the thickness of the metallized layer of the embossed media, e.g. metallized paper, has been reduced while simultaneously reinforcing the fibers to such an extent that the favorable folding properties have sensibly deteriorated.
In the operation of the cited devices, it is observed that the paper nerves of embossed papers having stronger fibers are no longer sufficiently broken by conventional embossing techniques, and that after the folding process, a so-called memory effect is observed as the folded portions tend to return to their original condition. The insertion of cigarette packaging units wrapped up in the new packing materials is thereby impaired or sometimes impossible. The above-mentioned effect further increases if the embossing paper loses humidity due to inadequate storage, thereby further increasing the strength of the paper fibers. This could be encountered by thermal treatments and controlled humidification of the media prior to folding resp. embossing, but for the packaging of foods, chewing gums, cigarettes, etc., it is not advantageous for hygienic reasons. In some cases, however, it may be advantageous if the embossed paper is smoothened by a subsequent roll.
In order still to be able to process such difficultly foldable materials, the contact pressure of the embossing rolls used according to the cited references is sometimes substantially increased as compared to the previously used standard media. However, the result is an important reduction of the tool life of the embossing rolls, and besides the costs for replacement parts, the necessary maintenance requires an interruption of the production, thereby reducing the overall efficiency and thus the productivity.
A parallel development is that in the quality inspection of the end users, the tolerance range of the media delivered by the paper manufacturers has been substantially restricted. The reason is that the embossing pressure, i.e. the contact pressure of the mating roll on the driven roll, has to be substantially further increased in order to always ensure a sufficient stamping if the variations in thickness are too large. Due to the higher demands in quality, the number of possible suppliers has been strongly limited, resp. the manufacture of the paper becomes more complicated.
It is the object of the present invention to improve the quality of the embossing and calendering as well as the folding behavior of the processed foil over the prior art without increasing the embossing pressure and thus the wear of the embossing rolls. This object is attained by a device wherein the second embossing roll is followed by at least another embossing roll which cooperates with the first or the preceding embossing roll and between which the patterned flat material is passed in order to receive essentially the same pattern in a re-embossing procedure.
According to the solution disclosed in claim 1, it has been found surprisingly that the presence of at least one following embossing roll for re-embossing the material not only allows a substantial reduction of the contact pressures, e.g. by half, but also a substantial improvement both of the optical properties of the embossed materials and of their folding properties.
Further embodiments and improvements of the device of the invention are described in the dependent claims.